WASHINGTON (Reuters) - The Afghan government lost control or influence of nearly 5 percent of its territory between January and May, the U.S government's top watchdog on Afghanistan said in a report on Friday, an indication of the challenges its forces are facing.

Fifteen years after the United States invaded Afghanistan to topple the Taliban rulers who had harbored al Qaeda militants who attacked the United States, the Taliban have made major gains and are estimated to control more territory than at any time since 2001.

Washington has been training and equipping Afghan security forces in order to withdraw America troops from the country, but the Afghans remain short of personnel and hardware.

The report, published by the Special Inspector General for Afghanistan Reconstruction (SIGAR), said the area under Afghan government "control or influence" had decreased to 65.6 percent by the end of May from 70.5 percent near the end of January, based on data provided by U.S. forces in Afghanistan.

That translates to a loss of 19 of the country's approximately 400 governing districts.

The report cited U.S. forces in Afghanistan as saying the loss of control was because Afghan forces were redeployed from lower-priority areas to "conduct offensive operations, gain and maintain the initiative, exploit opportunities, and consolidate tactical gains."

The commander of U.S. Forces in Afghanistan, Army General John Nicholson, said most of the areas the Taliban control were rural.

"They believed they were going to be able to seize and hold terrain, and they failed to do so," Nicholson told a Pentagon briefing via video link on Thursday.The report, however, said insurgents had 10 additional districts under their control or influence in the same timeframe. It was not possible to reach the Afghan government immediately for comment.

Acknowledging that security in Afghanistan remained precarious and Taliban forces had gained ground in some places, President Barack Obama shelved plans to cut the U.S. force in Afghanistan nearly in half by year's end, opting instead to keep 8,400 troops there through to the end of his presidency in January.

Obama also approved U.S. forces new authorities that enable them to accompany Afghan forces, while allowing greater use of U.S. air power.

Previously, Nicholson - who commands both the NATO-led Resolute Support mission and a separate U.S. counterterrorism mission - was permitted to take action against the Taliban only "in extremis," or when U.S. assistance was necessary to prevent a significant Afghan military setback.

A report by the Center on International Cooperation commissioned by the United Nations and published earlier this year found that the government had access to 61 percent of districts in 2015, down from 67 percent in 2010.

"The deficiencies and incapacities of the Afghan security forces have really not been addressed enough to avert such a scenario (of losing territory) in the future," said Michael Kugelman, a South Asia specialist at the Woodrow Wilson Center, a Washington think-tank.